1. Field of the Invention
The present invention relates to a loudspeaker system, and more particularly relates to a loudspeaker system which adjusts the acoustic pressure level of a loudspeaker, and simultaneously provides protection against a large input.
2. Description of the Related Art
Some conventional loudspeaker systems include a protection circuit which prevents damage due to an excessive input to a loudspeaker (refer to Japanese Laid-Open Patent Publication (Kokai) No. H8-33087). Some loudspeaker systems including tweeters and the like are provided with attenuator circuits for adjusting the acoustic pressure level of respective loudspeakers. FIG. 3 shows an example of the configuration of a conventional loudspeaker system.
The conventional loudspeaker system shown in FIG. 3 is provided with input terminals 10, a loudspeaker unit 20, an attenuator circuit 30 used for adjusting the acoustic pressure level, and an excessive input protection circuit 40. The attenuator circuit 30 includes resistor elements 31, 32 connected serially with each other, and a switch 33 which selectively switches between connected and disconnected states of two points including the resistor elements 31, 32 therebetween such that the acoustic pressure level output from the loudspeaker unit 20 is adjusted to a desired level by short-circuiting terminals 33a, 33b, or 33c using a jumper wire JP. The excessive input protection circuit 40 includes an over-current protection element 41 and a resistor element 42, and the resistor element 42 (shunt resistor) is connected in parallel with the over-current protection element 41. The over-current protection element 41 is a resistor element having a positive temperature coefficient, for example; it generates heat if a current more than a predetermined magnitude is passed, which rapidly increases the resistance of the resistor element, and consequently interrupts a signal. The resistor element 42 serves to restrict a signal current if the over-current protection element 41 interrupts the signal current, and supply the loudspeaker unit 20 with the resulting restricted signal to prevent a complete interruption of a reproduced sound output from the loudspeaker unit 20.
As described above, since the attenuator circuit and the excessive input protection circuit are separately provided in the conventional loudspeaker system, there is a problem that the number of resistor elements in these circuits increases, resulting in an increased cost. Further, in the conventional loudspeaker system, even if the attenuator circuit 30 attenuates the input signal to a sufficiently low level, namely, the attenuator circuit 30 is set to sufficiently attenuate the maximum possible signal applied to the input terminals 10 (set to attenuate the signal to a level equal to or less than the maximum rated input of the loudspeaker unit 20), since the signal is always supplied to the loudspeaker unit 20 through the over-current protection element 41, the sound quality often degrades due to a characteristic of the over-current protection element 41.